An engine lubricating system of an internal combustion engine such as a diesel engine is usually such that oil is drawn from an oil pan to an engine-driven oil pump and is circulated to the power producing and valve drive mechanisms of the engine through a filter unit provided downstream of the pump. An engine lubricating system of this nature is shown in, for example, "Weekly Service Bulletin" (page 345), issued October 1977 by Nissan Motor Company. The filter unit used in such an engine lubricating system is adapted to remove relatively large particles of, for example, casting sand and swarf from the oil passed therethrough. The filter unit thus has a filter medium having perforations measuring about 15 to about 20 microns in width, length and/or diameter. If particles of carbon produced by the combustion of fuel in the diesel engine and measuring about 5 angstrom to about 10 microns in diameter are mixed with the oil delivered from the oil pump, the particles are permitted to pass through the filter unit and to reach the surfaces of the various movable and/or rotatable members and elements such as the timing cams and the rocker arms forming part of the valve drive mechanism of the engine. The carbon particles deposited on these members and elements promote abrasion of the members and elements and thus result in creation of noises and vibrations and in failure of the intake and exhaust valves to open and close at correct timings. If the filter unit of the engine lubricating system is designed to be capable of collecting carbon particles, the filter medium of the unit tends to be clogged and would fail to function properly as filter means. The present invention contemplates resolution of these problems which have thus far been encountered in engine lubricating systems of internal combustion engines.